A typical LCD has the advantages of portability, low power consumption, and low radiation. LCDs have been widely used in various portable information products, such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
Referring to FIG. 5 and FIG. 6, a typical LCD panel 1 includes a color filter (CF) substrate 110, a thin film transistor (TFT) substrate 120 opposite to the CF substrate 110, and a liquid crystal layer 130 sandwiched between the two substrates 110, 120. The LCD panel 1 further includes a gate driver (not shown) and a data driver (not shown) disposed on the TFT substrate 120.
The CF substrate 110 includes a first glass substrate 111, a common electrode layer 112 formed on the first glass substrate 111 generally facing toward the liquid crystal layer 130, and a first alignment film 113 formed on the common electrode layer 112.
The TFT substrate 120 includes a second glass substrate 121, a number of gate lines 141 and a number of common lines 123 formed on the second glass substrate 121, an insulating layer 122 formed on the second glass substrate 121 covering the gate lines 141 and the common lines 123, a number of data lines 142 formed on the insulating layer 122, and a number of thin film transistors (TFTs) 140 formed on the insulating layer 122. The data lines 142 and the common lines 123 are parallel to each other and each data line 142 extends along a first direction. The gate 141 lines are parallel to each other and each gate line 141 extend along a second direction orthogonal to the first direction. The TFTs 140 are formed in the vicinity of intersections of the gate lines 141 and the data lines 142.
The TFT substrate 120 further includes a plurality of pixel electrodes 124 formed on the insulating layer 122, and a second alignment film 125 adjacent to the liquid crystal layer 130.
Each TFT 140 includes a gate electrode 1401 connected to the corresponding gate line 141, a source electrode 1402 connected to the corresponding data line 142, and a drain electrode 1403 connected to a corresponding one of the pixel electrodes 124.
FIG. 7 is an equivalent circuit diagram of one pixel unit of the LCD panel 1. Liquid crystal material sandwiched between the pixel electrode 124 and the common electrode layer 112 on the second glass substrate 121 define a liquid crystal capacitor Clc. Cgd is a parasitic capacitor formed between the gate electrode 1401 and the drain electrode 1403 of the TFT 140. Cst is a storage capacitor formed between the pixel electrode 124 and the common line 123.
When the LCD panel 1 works, at each pixel unit, an electric field generated between the pixel electrode 124 and the common electrode layer 112 is applied to the liquid crystal material of the liquid crystal layer 130. Light from a light source such as a backlight transmits through the TFT substrate 120, the liquid crystal layer 130, and the CF substrate 110. The amount of the light penetrating the LCD panel 1 at each pixel unit is adjusted by controlling the strength of the electric field, in order to obtain a desired optical output for the pixel unit.
If the electric field between the pixel electrode 124 and the common electrode layer 112 continues to be applied to the liquid crystal material in one direction, the liquid crystal material may deteriorate. Therefore, in order to avoid this problem, pixel voltages that are provided to the pixel electrode 124 are switched from a positive value to a negative value with respect to a common voltage. This technique is referred to as an inversion drive method.
FIG. 8 is an abbreviated timing chart illustrating operation of the LCD panel 1. In the chart, the x-axis denotes time, and the y-axis (not shown) denotes voltage. Von denotes a gate-on voltage provided by the gate driver for switching on the TFT 140. Voff denotes a gate-off voltage provided by the gate driver for switching off the TFT 140. Vd denotes a number of gradation voltages provided by the data driver. Vp denotes a number of pixel voltages of the pixel electrode 124. Vcom denotes a common voltage of the common electrode layer 112 and the common line 123 provided by an external circuit (not shown).
When a gate-on voltage Von is provided to the gate electrode 1401 of the TFT 140 via the corresponding gate line 141, the TFT 140 connected to the gate line 141 turns on. At the same time, a gradation voltage Vd generated by the data driver is provided to the pixel electrode 124 via the data line 142 and the activated TFT 140 in series. The potentials of the common electrode layer 112 are set at a uniform potential Vcom. Thus, an electric field is generated by the voltage difference between the pixel electrode 124 and the common electrode layer 112. The electric field is used to control the amount of light transmission of the corresponding pixel unit.
When a gate-off voltage Voff is provided to the gate electrode 1401 of the TFT 140 via the corresponding gate line 141, the TFT 140 turns off. The gradation voltage Vd applied to the liquid crystal capacitor Clc while the TFT 140 was turned on should be maintained after the TFT 140 turns off. However, due to the parasitic capacitance Cgd between the gate electrode 1401 and the drain electrode 1403 of the TFT 140, the gradation voltage Vd applied to the pixel electrode 124 is distorted. This kind of voltage distortion is known as a feed-through voltage VFD, and the feed-through voltage is obtained by the following formula (1):
                              V          FD                =                                            C              gd                        ⁡                          (                                                V                  on                                -                                  V                  off                                            )                                                          C              gd                        +                          C              lc                        +                          C              st                                                          (        1        )            
The voltage distortion VFD always tends to reduce the pixel voltage Vp regardless of the polarity of the data voltage, as shown in FIG. 8.
In an ideal LCD panel 1 as shown by a dashed line Vd in FIG. 8, when the gate-on voltage Von is provided to turn on the TFT 140, the gradation voltage Vd is applied to the pixel electrode 124, and thereby, when the gate-off voltage Voff is provided to turn off the TFT 140, the applied gradation voltage Vd should be maintained as the pixel voltage. But in an actual LCD panel 1 as shown by a solid line Vp in FIG. 8, when the gate-off voltage Voff is provided to the TFT 140, the pixel voltage Vp is reduced by the feed-through voltage VFD. 
An actual value of the voltage supplied to the liquid crystal material is obtained from the area between the pixel voltage Vp line and the common voltage Vcom line in FIG. 8. In one time frame, the pixel voltage Vp is greater than the common voltage Vcom, and this area can be considered to be a ‘positive’ area. In an adjacent frame, the pixel voltage Vp is less than the common voltage Vcom, and this area can be considered to be a ‘negative’ area. When the LCD panel 1 is driven by an inversion drive method, the level of the common voltage Vcom must be adjusted to keep the positive area of the one frame equal to the negative area of the adjacent frame. Therefore, a common voltage Vcom satisfying the above-mentioned condition needs to be supplied to the common electrode layer 112 in order to suppress the so-called flicker phenomena of a display screen of the LCD panel 1.
Generally, a dielectric constant of the liquid crystal material increases with increasing of the gradation voltage Vd provided to the liquid crystal layer 130. In other words, a capacitance of the liquid crystal capacitor Clc of each pixel unit changes according to the gradation voltage Vd provided to the corresponding pixel electrode 124. Thus the feed-through voltage VFD calculated according to formula (1) is changed with the gradation voltage Vd provided to the corresponding pixel electrode 124. Therefore a constant common voltage that can make the above-described positive and negative areas equal to each other is difficult to attain because of the changing of the feed-through voltage VFD.
It is desired to provide an LCD which can overcome the above-described deficiencies.